1). Field of the Invention
Embodiments of this invention relate to a method and apparatus for photolithographic imaging of substrates, such as semiconductor substrates.
2). Discussion of Related Art
Integrated circuits are formed on semiconductor wafers. The wafers are then sawed (or “singulated” or “diced”) into microelectronic dice, also known as semiconductor chips, with each chip carrying a respective integrated circuit. Each semiconductor chip is then mounted to a package, or carrier, substrate. Often the packages are then mounted to a motherboard, which may then be installed into a computing system.
Numerous steps may be involved in the creation of the integrated circuits, such as the formation and etching of various semiconductor, insulator, and conductive layers. Before the various layers may be etched, a layer of light-sensitive photoresist is formed on the substrate to protect the portions of the substrate that are not to be etched.
Machines referred to as photolithography steppers are used to expose the desired pattern in the photoresist layer. In order to achieve the desired pattern, light is directed through a reticle, or “mask,” and focused onto the substrate. Often the patterns on the masks, and thus the features in a particular layer on the substrate, are asymmetric in one direction. That is, the lines of the features may tend to be longer, or wider, in one direction, such as the “X-direction” in an X/Y coordinate system, than in the “Y-direction.”
Typically, the illumination methods used by the light sources in the steppers, such as annular, quadrupole, and hexa-pole, do not share the same asymmetric nature as the features on the mask. The resolution of the features that are exposed on the substrate is thus not maximized, and the size of the devices formed on the substrate is limited.
As the feature size required to be exposed by lithography continues to get smaller, it is necessary for the resolution of the lithography steppers, as well as the fidelity of the exposed structures, to improve.